Development of hot strength in synthetic magnesia refractories having high b{11 o content

ABSTRACT

A magnesia refractory shape possessing good strength properties at elevated temperatures is prepared from a magnesia grain containing 90 to 99% MgO, and over 0.1% by weight B2O3; and which can have a Ca-Si mole ratio of less than 2.0. Fine particles of an alkali metal-containing compound are added to the magnesia grain to help volatilize the boron during firing. Fine particles of calcium-and silicon-containing material can be admixed, if needed, with the magnesia grain to increase the Ca-Si mole ratio of the fine (-100 mesh) particles of the admixture to above 2:1 and to increase the total weight percent of CaO and SiO2 present in the fine particle component of the composition to between 3 to 7% by weight.

ilnited States Patent [1 1 Staut et al.

51 Sept. 3, 1974 [75] Inventors: Ronald Staut, Cherry Hill, N.J.;

Gunther L. Mort], Villach, Austria [73] Assignee: General Refractories Company,

Philadelphia, Pa.

22 Filed: Aug. 19, 1971 21 Appl. No.: 173,302

3,715,222 2/1973' Hieb 106/58 Primary ExaminerDe1bert E. Gantz Assistant Examiner-S. Berger Attorney, Agent, or FirmFinnegan, Henderson, Farabow & Garrett [5 7] ABSTRACT A magnesia refractory shape possessing good strength properties at elevated temperatures is prepared from a magnesia grain containing 90 to 99% MgO, and over 0.1% by weight B 0 and which can have a Ca-Si mole ratio of less than 2.0. Fine particles of an alkali metal-containing compound are'added to the magnesia grain to help volatilize the boron during firing. Fine particles of calcium-and silicon-containing material can be admixed, if needed, with the magnesia grain to increase the Ca-Si mole ratio of the fine (100 mesh) particles of the admixture to above 2:1 and to increase the total weight percent of CaO and SiO present in the fine particle component of the composition to between 3 to 7% by weight.

7 Claims, N0 Drawings It has been found that an excellent correlation exists between good hot strength characteristics and increased service life for basic refractories used in basic oxygen steel furnaces. Thus, the use of basic oxygen steel furnaces by steel companies has created a substantially increased demand for basic refractories having improved hot strength properties.

Refractory materials used in basic oxygen steel furnaces typically contain oxides of magnesium and calcium along with small amounts of the oxides of silicon, boron, iron and aluminum. Refractories which contain a relatively high percentage of calcium oxide tend to hydrate on exposure toair. Because of their reduced tendency to hydrate,.high magnesia content refractories, containing for example 96-98% MgO and relatively low amounts of calcium oxide, possess an inherently longer storage life than compositions containing a higher percentage of calcium oxide.

At the present time, high hot strength magnesia refractory compositions are conventionally made with a low flux magnesia grain having a B content less than 0.1% and preferably less tnan 0.05%, and with a CaO/- SiO mole ratio in the range of 2:1. The use of such a magnesia grain as a starting material is desirable because the presence of more than 0.05% by weight of B 0 is detrimental to the hot strength of synthetic magnesia compositions and because CaO/Si0 mole ratios of about 2:1 promote the formation of dicalcium silicate, a material with good refractory-properties that forms into isolated pockets rather than a continuous matrix when fired as part of a mix containing over 90% MgO. Thus, in a high purity fired magnesia'composition, the presence of dicalcium silicate will promote. particle to particle contact between MgO particles, a relationship that results in good hot strength since there is no flux between the load bearing particles to melt and create a weak spot at elevated temperatures.

It has now been found in accordance with the present invention, that a grain containing a high percentage of MgO, a relatively high B 0 content (greater than 0.1%), and a range of CaO/Si0 mole ratios including a relatively low CaO/Si0 ratio, for example below about 2 to 1, can be processed to yield a high hot strength refractory material.

The invention thus provides an improved process of preparing a magnesia refractory material possessing good strength properties at elevated temperatures. Specifically, the process comprises admixing fine particles of an alkali metal-containing compound with a low Fe O and A1 0 content magnesia grain starting material, containing 90 to 99% of MgO and over 0.1% by weight B 0 and firing the admixture to volatilize the alkali metal compound and the boron.

Forreasons of economy and ready commercial availability, the starting material preferably has a Ca-Si mole ratio of less than 2.0, and fine particles of calcium and silicon-containig compounds are admixed with the magnesia starting material to increase the CaO-Si0 mole ratio of the fine particle component of the composition to at least 2.0 and to increase the aggregate weight percent of Ca and Si, on an oxide basis, to between 3 to 7% by weight of the fine particle component of the composition. I

Unexpectedly, a brick or other refractory product produced from a high-boron-content magnesia grain in accordance with the process of the invention is almost indistinguishable from a similar refractory product produced from a more expensive, low-boron-content magnesia grain. After firing, there is almostno'trace of the alkali metal compound, and the boron content can genv erally be reduced to a level comparable to that of the usual commercially available so-called low boron magnesia grains. The strength properties of the fired refractory products produced by the invention'are excellent, and do not appear to be adversely affected by the volatilization of the alkali metal and boron during the firing step. i

" Magnesia starting materials having a B 0 content above 0.1% by weight and a low CaO/Si0 ratio, such as conventionally utilized in direct bonded refractories, can in accordance with the invention be used in preparing refractory shapes for basic oxygen steel furnaces. Prior to the present invention, two types of magnesia starting materials had to be stockpiled by refractorymanufacturers, one a relatively expensive low B 0 grain containing about 0.02% of boron on an oxide basis and having a CaO/SiO ratio of about 2:1 for use in basic oxygen furnaces, and the other a less expensive magnesia starting grain for bonded refractories having a CaO/SiO ratio of about 1:1, and containing considerably higher percentages of B 0 Now the latter starting material can be used for both purposes with considerable cost savings.

The eliminationiof the need for a low B 0 startingmaterial permits using a less expensive starting mate rial, results in production cost savings with respect to inventory requirements, and also eliminates contamination in magnesia grinding systems, and the cleaning cost involved in changing over from one magnesia grain to another. a

The invention resides in the novel process, produucts, compositions, and improvements shown and described. Both the foregoing general description and the following detailed description are exemplary, and should not be considered to restrict the scope-of the invention.

' available synthetic magnesiav grains. The invention is particularly well adapted tothe use of a starting magnes'ia grain containing to 99% MgO and over 0.1% by.

weight B 0 and having a CaO to SiO mole ratio less than 2.0. Such a synthetic magnesia grain is commonly available and is conventionally utilized in direct bonded refractories designed for less severe environments than basic oxygen steel furnaces. The chemical composition of a typical starting magnesia grain, in percent by weight, is 0.58% SiO 0.30% Fe O 0.23% Al- 0 0.75% CaO; 97.8% MgO; 0.14% B 0 and volatiles lost on ignition 0.19%.

The batch mix used in the present process should contain a blend of particle sizes that will give a closely packed mass. Preferably, the batch mix used in forming a refractory brick in accordance with the process of the invention contains the particles of the three usual size In accordance with the invention, an alkali-metal pounds as set forth above, the resulting admixture is 1 formed into the desired shape by anypractical method of consolidation such as tamping, camming, vibratory compaction or by pressing. Pressures of about 5,000 to containing compound is added to the magnesia starting 5 20,000 psi are usually used to obtain the desired material to helpvolatilize the boron present in the magpressed densities. After forming, the refractory shapes nesia grain. The. alkali metal containing compound are fired at temperatures of from 2800 to 3200F for ShOuld be an oxide, or a mp Which is ily from 1 to hours. Preferred firing temperatures converted to the oxide form upon firing,.such as a nirange from 3000 to 32O0F.

trate, acarbonate,orahydroxide. Typical alkalimetal- 10 The refractory materials produced by the present containing compounds which can be used in the presprocess possess desirable hot strength properties and ent invention include KOI-I, NaOH, K CO Na CO comprise after firing 90 to 98% by weight of MgO in KNO NaNO K 0, Na O, and the corresponding liththe form of tightly packed MgO' particles predomiium compounds. nantly in particle-to-particle contact. A discontinuous The amount of alkali metal-containing compound silica-containing phase comprises isolated pockets of which is admixed with the starting magnesia grain Ca S. The hot crushing strengths of refractory articles should be sufficient to reduce the B 0 content of the of this invention are generally above 4500 psi, and frefired composition to below 0.05% by weight, usually quently many test greater than 5000 psi. the alkali metal compound. is added in amountsto pro- For a better understanding of the invention, the folvide 0.2l% and preferably 04-06% by weight of allowing examples are provided. These examples are inkali metal on anoxide basis in the admixed compositended to be illustrative and should not be construed as tion. Preferably, the alkali metal compound is in finely limiting the invention. All parts and percentages listed divided form because uniform distribution of the alkali in h P fiCa OH and Claims are by Weight unless 0thmetal throughoutthe composition helps promote unieYWlSe noted- All Screen h deslgnatlons are form volatilization of boron fromthe composition dur- Standard Screen Unless Othefwlse noted ing firing.

If the magnesia grain selected as a starting material, Th 1 i ;h h h b has a CaO to SiO mole' ratio of less 2.0 with respect to ese P es 5 PW 0W 0t ff can i: fineslcgmgonergtcgparticles belowl l(f) 0 mesh) or, if gi a gigrd mtlliinag s;alr(t):;lgC 2 1 gt/P1;l(a)l cmogisrlirgg 2315:1152? t e tota a i 2 content 0 t e me raction is 2 below 3%, then it is desirable to admix with the starting tammg a h Welght P f of boron Over material fine particles of calcium and/or silicongi fi xgi g f gg ggg g g; 2,23 gi f g g gfg containing compounds to achieve these values. The I 0 f Ca/Si mole ratio of the fines and the overallweight peri g r CT? .8 cent of CaO and SiO present in the fines, must be 2 i Stilr mg 3, 1 5 equal or above these values to Obtain high hot z n fains di stributio f pgr tic l s i s to prod iic e a strength properties in the resultant refractory product. Closely packed mass. It contains 65% y Weight of l 4 Th I X 35 mesh material and 35% of a fine fraction in which e calcium and/or silicon-containing compounds 40 b dd d b f ft th fth 80% by weight is minus 325 mesh. a el l e Ore f l e 1 Ion The CaO/Si0 weight ratio of the fine fraction is inl %t 5 3 lg 9 creased to the amounts shown in Table I for each samt e f q a g i g 2 2 I 2 ple by addition of wollastonite (CaSiO and CaCO as a mo e ratio 0 to to to pre y 3 to -325 mesh particles. Sufficient '325 mesh fines of Na to and m an aggregate amount equa to 0 y CO are then added to the samples to provide one perweightof theorigmal fines, preferably 3.55.5%- cent by weight of sodium on an oxide basis in each A widedvarietybof ctall ciuntand/or SlllCiJfil-CSHSI/lglgg composition COmPOUn 5 can 6 0 Increase e a 1 The cdm ositions are s'ha ed into c linders 1 inch in m ratio and the Weight Percent 9 these Compounds height and l /s inch in diam eter and ressed a 15,000 I h refractory WPQ I Sultable 9 of psi and fired to 3 1 70F and held at that temperature for ma lnclude WOllflStOllltC (CaS Q serpentine, diopside 3 hours. d 2 or gh 2 cohtalhmgmaghesfai Sultable Table I below shows the flux content of the fine fracsources of CaO include wollastonite, calcium carbontion, the CaO/SiO weight ratio of the fine fraction, fiteilclcum hydr0X1de, l 11 Calclum filtrate, and green bulk density, fired bulk density, open porosity 1g a Contamlng gh and 2800F hot crushing strength of the compositions The alkali mtal-contaihmg Compound andbthe Calclto which Na O is added. The hot crushing strength reumand silicon-Containing compo n a 6 Convefleets the force necessary to crush the fired cylinder niently added to.the magnesia starting grain in a suitwhen load is applied along the cylindrical axis. In Table able mixer. After the addition of the alkali metal coml, the flux content reflects the weight percent of CaO pound, and if necessary the silicon and calcium comand Si0 present in the fines.;

TABLE I HOT 1 GREEN FIRED FLUX cimgn EX.. BULK BULK C/S CONTENT STRENGTH N0; DENSITY DENSITY POROSITY RATIO IN FINES (PSI) 6 TABLE I Continued 2800F 70 HOT GREEN FIRED FLUX CRUSH ING 15X. BULK BULK C/S CONTENT STRENGTH NU DENSITY DENSITY POROSITY RATIO IN FINES (PS1) 3 2.82 2.80 19.8 2.5 4.0 5000+ 4 2.84 2.80 19.6 3.0 4.0 4632+ 5 2.83 2.80 19.8 3.5 4.0 5000+ 6 2.81 2.73 20.2 4.0 4.0 4950+ 7 2.82 2.79 20.1 4.5 4.0 4563+ is 2.113 2.110 19.7 5.0 4.0 4831+ I 2.115 2.82 19.4 2.0 4.5 4983+ 2.110 2,77 207 2.5 4.5 4675+ l 1 2.111 2.77 207 3.0 4.5 4920+ 12 2.7! 2.140 19.8 3.5 4.5 4720+ 13 2,110 2.76 20.9 4.0 4.5 4970+ 14 2.81 2.76 20.8 4.5 4.5 4408+ 2.82 2.77 20.5 5.0 4.5 4213+ 16 2.76 2.76 21.2 2.0 5.0 4941+ 17 2.77 2.75 21.4 2.5 5.0 4563+ 18 2.75 2.74 21.8 3.0 I 5.0 5000+ 19 2.75 2.74 21.8 3.5 5.0 4900+ 2.75 2.74 21.7 4.0 5.0 4903+ 21 2.78 2.74 21.5 4.5 5.0 4406+ 22 2.79 2.75 21.3 5.0 5.0 4380+ Another series of refractory materials is prepared utilizing the techniques described above with respect to content and a low CaO/Si0 mole ratio in accordance with the present invention.

The foregoing detailed description has been provided for cleamess of understanding only, and no unnecessary limitations should be implied therefrom. Some modifications of the process and product described may be readily apparent to those skilled in the art.

TABLE II 2800F HOT GREEN FIRED FLUX CRUgH- IN CONTROL BULK BULK C/S CONTENT STRENGTH TEST NO. DENSITY DENSITY POROSITY RATIO IN FINES (PS1) The improvement in 2800F hot crushing strength achieved through the incorporation of alkali metal oxides in the batch mix which is fired to produce magnesia refractories is dramatically shown by comparing the results of Examples l-22 tabulated in Table I with the corresponding series of tests in which no alkali metal oxide was used, as shown in Table II. The hot crushing strength of the products produced by the invention is markedly greater than the control composition over the entire range of the tests. The strengthincreases by a. factor of 10 at CaO/Si0 ratios between 2 and 3, and by a factor of about 2 at higher CaO/Si0 ratios.

The results of Examples l-22 thus clearly demonstrate the production of high hot strength refractory compositions frommagnesia grains having a high B 0 What is claimed is:

l. A process of preparing a shaped magnesia refrac-' calcium-containing compound and, if necessary, of

a silicon-containing compound, the amount of calcium-containing compound, and any siliconcontaining compound added to the magnesia grain starting material being sufficient to increase the Ca-Si mole ratio of the particles below 100 mesh present in the composition to between 2 to l and 5 to l, and to increase the total weight percent on an oxide basis of Ca plus Si of the particles below 100 mesh in the admixture to between 3 to 7% by weight;

forming the admixed composition to produce a shaped article; and

firing the shaped article to produce a shaped product.

2. The process of claim 1 in which the materials admixed with the magnesia grain are all 325 mesh.

. 3. The process of claim 1 in which fine particles of wollastonite and CaCO are added to the magnesia starting material contains about 98% by weight MgO. 

2. The process of claim 1 in which the materials admixed with the magnesia grain are all -325 mesh.
 3. The process of claim 1 in which fine particles of wollastonite and CaCO3 are added to the magnesia grain starting material as sources of silicon and calcium.
 4. The process of claim 1 in which sodium carbonate is added to the magnesia grain starting material as a source of alkali metal.
 5. The process of claim 3 in which sodium carbonate is added to the magnesia grain starting material as a source of alkali metal.
 6. The process of claim 1 in which sufficient alkali metal-containing compound is admixed to provide 0.2-1% of sodium, on an oxide basis in the admixed composition.
 7. The process of claim 1 in which the magnesia grain starting material contains about 98% by weight MgO. 